Membranes are porous solid materials and are used for separating mixtures. One advantage of membrane separation processes is that they can be carried out even at low temperatures, such as room temperature for example, and therefore, compared with thermal separation processes, such as distillation, sublimation or crystallization, have lower energy requirements and the mixture to be separated suffers less thermal stress. Moreover, separations are achievable with the aid of membranes that are not possible with thermal processes because, for example, the presence of azeotropes and of crystal isomorphs rule out respectively a distillative separation and a separation by recrystallization. However, there are issues with limitations concerning the useful life, the separation performance, the flux rates as well as the mechanical properties of membranes and also the costs associated with their production.
Membranes based on polyvinyl alcohols (PVA) or polyvinyl acetates (PVAc) are well known. PVA membranes are used for micro- and ultrafiltration (Chuang, Polymer, 2000, Volume 41, pages 5633 to 5641). JP-A 3030820 describes the production of syndiotactic polyvinyl alcohols from vinyl acetate and optionally further monomers such as vinyloxytrimethylsilane and the use of such polyvinyl alcohols for producing membranes. EP-A 423949 recommends the use of PVA-based membranes for separating azeotropic water-ethanol mixtures. Separation of this type is also known as pervaporation. PVA is particularly useful as membrane material for separating water mixtures owing to its hydrophilic polymeric scaffold. Membrane separation processes applied to water-ethanol mixtures produce 99% by weight pure ethanol. PVA further has a low tendency to adsorb protein, making it possible to reduce membrane fouling, i.e., the accumulation of organic impurities on the membrane.
The separation-selective PVA layer can be situated on a porous support structure, or PVA itself can be made into a porous membrane, for example using phase inversion processes. EP-A 96339 describes PVA membranes which are applied to porous polyacrylonitrile support structures. GB-A 1325227 describes porous PVA membranes produced by the phase inversion process.
In the course of being used to produce PVA membranes, the polyvinyl alcohol used for this purpose is generally subjected to a crosslinking step. This can take the form, for example, of acetalization with dialdehydes, such as glutaraldehyde. Membranes thus obtainable, however, only have a very limited stability, since corresponding acetals can hydrolyze under both acidic and basic conditions, which appreciably limits the usefulness of corresponding membranes with regard to the pH values of the solutions to be separated. Alternatively, crosslinking of polyvinyl alcohol can be effected using diisocyanates or anhydrides, for example maleic anhydride. But even this approach leads to membranes of inadequate stability. Another disadvantage is that the aforementioned crosslinking reagents are poisonous and can only be handled under costly and inconvenient safety precautions on a large industrial scale. Moreover, the PVA membranes thus obtainable swell substantially at temperatures above 70° C. and/or in solutions having high water contents, which impairs the stability, the water flow rate and the selectivity of PVA membranes.
Yet it is specifically when membranes are used to separate mixtures having high water contents that employing temperatures of above 70° C. makes it possible to enhance the space-time yields and the process efficiency. To make this possible, for example, the production of PVA membranes by crosslinking of PVA with tetraethyl orthosilicate is recommended (Karin Kul, Applied Polymer Science, 2004, Volume 94, pages 1304 to 1315). However, even PVA membranes thus obtainable are still not sufficiently stable to hydrolysis. In addition, the membranes are very costly and inconvenient to produce, since the individual starting materials for producing the PVA membrane are usually immiscible, so that a phase separation occurs between PVA and silicate and finally membranes of inhomogeneous composition are obtained, which are unsatisfactory with regard to their swelling behavior and their stability.